The present invention relates to self-elevating jack-up rigs for offshore oil exploration and production, and more particularly to a system utilizing an improved rack chock assembly for support legs.
The term "Jack-Up Rig Unit" is used in the art to identify a working platform adapted for conducting drilling, workover, production and other offshore operations. Such platforms are supported in an elevated position above the water line by a number of jackable legs which are adapted to move relative to a hull and allow towing of the platform to a desired location with the legs elevated above the hull. Once on the location, the legs are lowered to the bottom of a body of water, while supporting the hull in its desired elevated position.
One of such rigs and associate leg/hull rack chock system is disclosed in my earlier U.S. Pat. No. Re. 32,589, re-issued on Feb. 2, 1988, a full disclosure of which is incorporated by reference herein. That patent disclosed a method and apparatus for rigidly supporting the jack-up unit in an elevated position on the legs of the unit and/or for rigidly supporting the legs in a raised position when the unit is afloat. In accordance with the disclosure of that patent, rack teeth of the supporting leg are engaged with opposed, matching rack sections of a rack chock which can be moved for adjustment vertically up and down, as well as laterally in and out of engagement with the leg chord. The rack chock system of U.S. Pat. No. Re. 32,589 proved to be structurally successful because it provides a system of interdigitation of the teeth on the rack chock and the legs which engages the hull with the legs without introducing any substantial bending moments on the legs.
However, there exists a certain difficulty in breaking the rigid engagement between the teeth of the rack chock and the legs to allow withdrawal of the rack chock and movement of the legs in relation to the hull, when the conditions so require. The horizontal screw jacks have a tendency, under certain conditions, to lock-up against the chock and require more torque than is available to initiate withdrawal of the screw jacks. It is believed that the locking-up of the chock pressing against the screw jack is caused by forces and moments in the legs, which result from the chock moving away from the rack when loaded. This is most likely to occur when the rig is elevated above the sea surface and experiences a major storm. Sometimes it occurs after a rig is afloat in heavy seas and the vessel experiences a great deal of pitching and rolling. The vertical screw jacks which move the rack chock up or down in relation to the legs do not experience this locking-up phenomenon under the above stated conditions.
As a result of the locking-up of the rack chock, the rig personnel looses valuable time in an attempt to free up one or more horizontal screw jacks. The usual procedure which is used now is to destroy a cap which is pressed against the chock by a horizontal screw and withdraw the screw jack and the rack. The cap is then replaced before the screw jack is moved laterally to move the chock into an engagement with the leg teeth.
The present invention contemplates elimination of the drawbacks associated with the undesirable locking-up of the chock and provision of an improved rack chock assembly where the locking-up phenomenon of the screw jacks against the chock is minimized or altogether eliminated.